Impedance matching network for use with sputtering apparatus

ABSTRACT

AN IMPEDANCE MATCHING NETWORK FOR USE WITH A RADIO FREQUENCY BIAS SPUTTERING SYSTEM WHERE THE SYSTEM IS IMPEDANCE MATCHED TO A RADIO FREQUENCY POWER GENERATOR CONNECTED VIA AN IMPUT LINE TO THE IMPEDANCE MATCHING NETWORK. THE MATCHING NETWORK PERMITS NEAR 0 TO 100% BIAS SPUTTERING IN EITHER DIRECTION.

' Nov. 28, 1972 R. B. MCDOWELL 3,704,219 IMPEDANCE MATCHING NETWORK FORUSE WITH SPUTTERING APPARATUS Filed April 7, 1971 INVENTOR ROBERT BRUCEMCDOWELL ATTORNEYS.

mozmmzmo m 1 United States Patent Office Patented Nov. 28, 19723,704,219 IMPEDANCE MATCHING NETWORK FOR USE WITH SPUTTERING APPARATUSRobert Bruce McDowell, Metuchen, N.J., assignor to McDowell Electronics,Inc., Metucben, NJ. Filed Apr. 7, 1971, Ser. No. 131,903 Int. Cl. C23c15/00 US. Cl. 204-192 13 Claims ABSTRACT OF THE DISCLOSURE An impedancematching network for use with a radio frequency bias sputtering systemwhere the system is impedance matched to a radio frequency powergenerator connected via an input line to the impedance matching network.The matching network permits near to 100% bias sputtering in eitherdirection.

BACKGROUND OF THE INVENTION This invention relates to sputtering systemsand, in particular, to impedance matching networks for use therewith.

The purpose of radio frequency bias sputtering is to support a certainamount of back sputtering during the forward sputtering process. Thissmall amount of back sputtering, or small percentage of RF biassputtering insures a cleaner sputtered material since all loosely bondedparticles are sputtered back off the substrate. The radio frequencyenergy from the stabilized radio frequency power supply must be appliedto the sputtering electrode assembly including an anode and cathodethrough an impedance matching network. Conventional bias sputteringsystems include two separate impedance matching networks, one for theanode and the second for the cathode. However, there is much difiicultyin tuning this type of system since the tuning of each network detunesthe other. Thus, many dials have to be tuned and retuned many times foreach level of radio frequency biasthat is, percentage of radio frequencybias, and for each change in electrode spacing or sputtering chamberpressure.

Further, conventional metering used with bias sputtering systems is thatused for measuring radio frequency power and voltage or current. Withthis type of metering, it becomes necessary to calculate the actualsheath voltage diiferential, which in reality is a negative directcurrent potential.

SUMMARY OF THE INVENTION A primary object of this invention is toprovide a unique single impedance matching network for matching a radiofrequency power supply to a sputtering electrode system.

A further object of the invention is to provide a unique impedancematching network of the above type having direct filtered D.C. meteringof each sheath voltage.

It is a further object of the invention to provide an impedance matchingnetwork of the above type which is easily tuned to effect the desiredimpedance match between the radio frequency power source and thesputtering electrode system.

It is a further object of this invention to provide a unique impedancematching network of the above type having an input circuit portion whichcan be either series or shunt tuned to an input line to the network ofany fixed characteristic impedance.

It is a further object of the invention to provide a unique impedancematching network of the above type having an output portion which can beseries or shunt tuned to the impedance of individual electrodes of thesputtering electrode system with respect to each other or with respectto ground.

It is a further object of this invention to provide a unique impedancematching network of the above type which enables the placement ofpositive DJC. sheath voltages on at least one of the electrodes of thesputtering electrode system whereby a plasma anodization type ofsputtering technique is available.

It is a further object of this invention to provide a unique radiofrequency bias sputtering system utilizing at least one positive D.C.sheath voltage for implementing a plasma anodizing kind of sputteringtechnique.

It is a further object of this invention to provide a unique impedancematching circuit of the above type wherein a circuit may be readilymodified to implement radio frequency diode sputtering.

Other objects and advantages of this invention will become apparent uponreading the appended claims in conjunction with the following detaileddescription and the attached drawing.

BRIEF DESCRIPTION OF THE DRAWING The figure of the drawing is aschematic diagram of illustrative circuitry constituting the uniqueimpedance matching network of this invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTIONReferring to the drawing there is shown an illustrative impedancematching network in accordance with the invention. This network may beconnected directly to a Radio Frequency generator 10 or connected to thegenerator 10 via a coaxial cable 12. The characteristic impedance ofcable or line 12 is typically 50 ohms but may be of any fixedcharacteristic impedance. Connected to cable 12 is Radio Frequency wattmeter 14, which may represent one or more watt meters to measure forwardand reflected power on cable 12.

The input terminal 16 of the impedance matching network is connected toan inductive voltage divider network generally indicated at 18comprising two parallel connected variable inductors 20- and 22 whichare respectively connected to the inner conductor 24 of the coaxialcable and the outer conductor 26 which is grounded as indicated at 28.

The inductors 20 and 22 are tuned to the input line 12 by means of ashunt input tuning capacitor 30. Broadly speaking, inductors 20' and 22and tuning capacitor 30 may be termed the input portion of the impedancematching circuit.

Adjustable taps 32 and 34 for variable inductors 20 and 22 are ganged sothat when the voltage from one of the inductors is at a maximum thevoltage from the other is at a minimum and vice versa with intermediatesettings of one inductor increasing whenever the intermediate settingsof the other decrease or vice versa. The taps 32 and 34 are respectivelyconnected to series tuning capacitors 36 and 38 which are respectivelyconnected to the anode 40 and the cathode 42 of the sputtering electrodeassembly diagrammatically indicated by the dotted line 44. A shutter 46is also illustrated and is connected to ground via conductor 48. Thevariable capacitors 36, which will be brought out in more detailhereinafter, allow differential sheath voltage adjustment in addition tothat obtainable from the variable inductors 20 and 22 whereby anydesired amount of radio frequency bias sputtering in either directioncan be obtained. Broadly speaking, the capacitors 36 and 38 may betermed the output portion of the impedance matching network.

Each of the sheath voltages at the anode 40 and catho'de 42 arerespectively measured by direct current meters 50 and 52. These metersare isolated by Radio Frequency 3 chokes 54 and 56 respectively from theanode 40 and cathode 42. Appropriate meter loading is elfected byswitches 58 and 60. Switch 58 selects the appropriate one of resistors62, 64 and 66 while switch 60 selects the appropriate one of resistors'68, 70 and 72 to effect the appropriate loading of the meters 50 and52.

In operation, the radio frequency power generator is properly tuned tothe characteristic impedance of coaxial or shielded line 12, asindicated by directional Radio Frequency watt meter 14. Afteraccomplishing this, the inductive voltage divider 18 is set at theapproximate position required for the desired amount of radio frequencybias and the desired sputtering direction. Capacitor 30 is then adjusteduntil the input portion of the impedance matching network is tuned tothe characteristic input impedance of line 12 as indicated on thedirectional watt meter 14 so that there is maximum forward power andminimum reflected power in line 12. Although the tuning is shown asshunt in the figure, it is to be understood that this tuning could alsobe series tuning whereby a pair of variable capacitors would berespectively connected in the lines including variable inductors 20 and22.

The desired sheath voltage of the Radio Frequency high electrode, aspredetermined by the setting of the taps of the inductive voltagedivider 18, is tuned in by one of the series capacitors 36 or 38.Assuming that the anode 40 is the high voltage electrode, the variablecapacitor 36 would be tuned until the desired sheath voltage is obtainedfor the anode. This would be indicated by the reading on meter 50. Thetuning adjustment of the high voltage electrode is critical and itssetting may require the returning of the input tuning capacitor 30, ifan increase in reflected line power is indicated in line 12.

The final adjustment is that of the low voltage electrode. Assuming thatthis electrode is the cathode 42, this is accomplished by adjustingtuning capacitor 38 to the desired sheath voltage as indicated by meter5-2. Thus, the desired percentage of radio frequency bias is obtained.Hence, if the sheath voltage of the anode were --1500 volts and that ofthe cathode were 300 volts, the percentage of radio frequency bias wouldbe 20%.

The adjustment of the low voltage electrode 42 is not critical and inmost cases will not necessitate the retuning of the input circuitportion. Assuming that capacitor 38 is employed to effect the tuning ofthe low voltage electrode, it is possible with this capacitor to obtainan extremely wide sheath voltage range even in the positive direction,the ramifications of which will be described in more detail hereinafter.

Although tuning of the anode 40 and cathode 42 has been described interms of the series capacitors 36 and 38, the tuning of these electrodescan also be effected by shunt tuning whereby first and second shuntcapacitors would be respectively connected from the lines connected totaps 32 and 34 respectively to ground.

Since it is clear either the anode or cathode may be selected as thehigh voltage electrode, sputtering may be effected in either direction.Further, since the sheath voltage dilferential can be roughlyestablished by the setting of taps 32 and 34 and very preciselyestablished by variable capacitors 36 and 38, the percentage of radiofrequency bias can also be accurately established and in fact, near 0 to100% bias sputtering can be effected in either direction withoutphysically or electrically changing wires or connections.

Further, this is accomplished through only four tuning dials (notshown), one of which would be connected to the ganged taps 32 and 34,the second of which would be connected to capacitor 30, the third ofwhich would be connected to capacitor 36 and the fourth of which wouldbe connected to capacitor 38.

A further aspect of the invention is that by a simple modification ofthe circuitry of the figure, the circuitry can be converted from oneimplementing radio frequency bias sputtering to one that implementsconventional radio frequency diode sputtering. This is done by merelyswitching switch 74 from the position shown in the figure to terminal 75thereby grounding the anode 40 and disconnecting the variable capacitor36 from the circuit. The tuning procedure for radio frequency diodesputtering would be the same as that for radio frequency biassputtering, as described hereinbefore, with the single exception thatthe variable capacitor 36 would not be tuned since it would, of course,not be a part of the circuit.

As stated hereinbefore, the tuning range of the capacitor associatedwith the low voltage electrode may be extremely wide and thus positivesheath voltages are available at this electrode. Thus, if capacitor 38is varied to establish the sheath voltage at cathode 42, it beingassumed that the cathode is the radio frequency voltage electrode, thecapacitor 38 may be so varied such as to establish a positive sheathvoltage at the cathode 42. Present anodizing processes are generallyeffected electrolytically in a liquid bath across which a DC. potentialhas been applied to produce free oxygen, the part to be oxidized orsubstrate being connected to the positive terminal. Negative oxygen ionsform on the substrate thereby oxidizing it.

With positive RF bias or DC sheath voltage on the cathode 42, thecathode or substrate electrode 42 will be positive with respect to theplasma. Hence, anodization of both electrically conductive andinsulative materials is possible. Further, with positive radio frequencybias voltage or DC. sheath voltage, insulating substrates can beanodized by the radio frequency induced anodizing voltage with no formsof conducting clips, as required in conventional methods of anodizing.

A further advantage in positive RF bias sputtering or positive D.C.sheath voltages appears to be present in the process of sputteringtantalum to insulating materials such as quartz or ceramics. Theadvantage would be better adhesion. During the cycle of positive sheathvoltage, tantalum oxide would be sputtered. When the substrate electrodeis returned to establish zero sheath voltage or negative sheath voltage,tantalum would then be sputtered, rather than tantalum oxide.

Numerous modifications of the invention will become apparent to one ofordinary skill in the art upon reading the foregoing disclosure. Duringsuch a reading it will be evident that this invention provides a uniqueimpedance matching network for accomplishing the objects and advantagesherein stated.

What is claimed is:

1. An impedance matching network for use in a radio frequency biassputtering system including first and second electrodes, said systembeing connected to a radio frequency power generator, said networkcomprising:

(a) an input circuit including first and second variable inductorsconnected in parallel across the output of said generator where firstand second taps are respectively connected to said first and secondvariable inductors to derive the respective variable voltages therefromand where said first and second taps are ganged together so that whenthe voltage output from said first variable inductor changes inmagnitude in one direction, the voltage output from said second variableinductor changes in magnitude in the other direction and at least onevariable capacitor for tuning said input circuit to the output impedanceof said generator;

(b) means connected to the variable output voltage terminal of saidfirst variable inductor and said first electrode for further varying thevoltage from said first variable inductor to thereby establish thesheath voltage at said first electrode; and

(c) means connected to the variable output voltage terminal of saidsecond variable inductor and said second electrode for further varyingthe voltage from said second variable inductor to establish the sheathvoltage at said second electrode;

whereby near to 100% bias sputtering can be effected in either directionbetween said first and second electrodes.

2. A network as in claim 1 where said first and second tuning means eachincludes a variable capacitor connected in series between its associatedinductor and electrode.

3. A network as in claim 2 where said one variable capacitor of saidinput circuit is connected across the said output of said generator.

4. An impedance matching network as in claim 1 including switching meansfor connecting said first electrode to (1) said means for establishing asheath voltage at said first electrode or (2) a reference potentialwhereby said impedance matching circuitry is switched to (l) a radiofrequency diode sputtering mode of operation when said first electrodeis connected to said reference potential and (2) a radio frequency biassputtering mode of operation when said first electrode is connected tosaid means for establishing the sheath voltage at said first electrode.

5. An impedance matching network for use in a radio frequency biassputtering system including first and second electrodes, said systembeing connected to a radio frequency power generator by an input line,said network comprising:

(a) an input circuit including first and second variable inductorsconnected in parallel across said input line where first and second tapsare respectively connected to said first and second variable inductorsto derive the respective variable voltages therefrom and where saidfirst and second taps are ganged together so that when the outputvoltage from said first variable inductor changes in magnitude in onedirection, the output voltage from said second variable inductor changesin magnitude in the other direction and at least one variable capacitorfor tuning said input circuit to the characteristic impedance of saidinput line;

(b) means connected to the variable output voltage terminal of saidfirst variable inductor and said first electrode for further varying theoutput voltage from said first variable inductor to thereby establishthe sheath voltage at said first electrode; and

(c) means connected to the variable output voltage terminal of saidsecond variable inductor and said second electrode for further varyingthe voltage from said second variable inductor to establish the sheathvoltage at said second electrode whereby near 0 to 100% bias sputteringcan be effected in either direction between said first and secondelectrodes.

6. A network as in claim 5 where said first and second tuning means eachincludes a variable capacitor connected in series between its associatedinductor and electrode.

7. A network as in claim 6 where said one variable capacitor of saidinput circuit is connected across said input line.

8. A network as in claim 5 where said first and second electrodes are ananode and a cathode respectively.

9. A network as in claim 8 including a first measuring circuit connectedbetween said anode and a reference potential for measuring the anodesheath voltage and a second measuring circuit connected between thecathode and reference potential for measuring the cathode sheathvoltage.

10. A network as in claim 9 where each said measuring circuit includes avoltage meter in series with a radio frequency choke.

11. A network as in claim 5 where said input line is a coaxial cable andwhere the outer conductor thereof is grounded.

12. A method of operating the network of claim 5 including the steps of:

(a) adjusting the position of the ganged taps of said first and secondvariable inductors until the desired sputtering direction and amount ofradio frequency bias is obtained;

(b) adjusting said one variable capacitor until said input circuit isimpedance matched to said input line;

(c) adjusting said means for establishing the sheath voltage at saidfirst electrode until said last-mentioned sheath voltage is established;and

(d) adjusting said means for establishing the sheath voltage at saidsecond electrode until said last-mentioned sheath voltage isestablished.

13. The method as in claim 12 where said means for establishing thesheath voltage at said second electrode is adjusted until a positivebias voltage with respect to ground is established at said secondelectrode.

References Cited UNITED STATES PATENTS 3,471,396 10/1969 Davidse 2042983,525,680 8/1970 Davidse et al. 204298 3,616,405 10/1971 Beaudry 204298JOHN H. MACK, Primary Examiner S. S. KANTER, Assistant Examiner US. Cl.X.R. 204298

